In sand blasting process, a workpiece is assembled to a fixture by a robot arm, and then the workpiece and the fixture are transported to a sand blasting apparatus. A suction assembly configured to grip the workpiece and a clamping assembly configured to clamp the fixture are positioned at two sides of the robot arm. After assembling the workpiece to the fixture, there is a need to turn the robot arm, and then the fixture with the workpiece can be clamped by the clamping assembly and transported by the robot arm.